Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Earth's Internal Processes
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Earth's Internal Processes
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Subject: Geography
Class: Senior Secondary 3
Term: 1st Term
Week: 1
Theme: The Earth And The Solar System
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This topic introduces teachers to the fundamental forces originating within the Earth that shape its surface. Understanding Earth's internal processes is crucial for Senior Secondary 3 students as it provides the foundation for comprehending geological phenomena such as mountain building, volcanic eruptions, and earthquakes. These processes significantly influence the distribution of natural resources, the formation of landscapes, and pose potential hazards, all of which have direct relevance to Nigeria's physical environment, resource base, and disaster preparedness strategies.
Symmetrical patterns of magnetic reversals in rocks on either side of mid-oceanic ridges, indicating new crust forms and spreads outwards.
Age of oceanic crust: Rocks are youngest near mid-oceanic ridges and progressively older further away. Global distribution of earthquakes and volcanoes: Concentrated along plate boundaries.
Types of Plate Boundaries:
1. Divergent Boundaries (Constructive): Plates move apart.
Features: Mid-oceanic ridges (e.g., Mid-Atlantic Ridge), rift valleys (e.g., East African Rift Valley), volcanism, shallow earthquakes. New crust is generated here.
Example in Nigeria's vicinity: The East African Rift System extends towards the Gulf of Guinea, representing an ongoing continental rifting process that could eventually lead to the formation of a new ocean basin. The Benue Trough in Nigeria is an ancient failed arm of a triple junction rift system.
2. Convergent Boundaries (Destructive): Plates move towards each other, resulting in collision or subduction. Crust is destroyed here.
Oceanic-Continental Convergence: Denser oceanic plate subducts beneath the continental plate.
Features: Oceanic trench, volcanic arc on the continent (e.g., Andes Mountains), strong earthquakes.
Oceanic-Oceanic Convergence: One oceanic plate subducts beneath another.
Features: Oceanic trench, island arc (e.g., Mariana Trench and Mariana Islands), strong earthquakes.
Continental-Continental Convergence: Two continental plates collide (neither subducts significantly due to similar density).
Features: Intense folding and faulting, formation of high mountain ranges (e.g., Himalayas), broad area of shallow to deep earthquakes.
3. Transform Boundaries (Conservative): Plates slide horizontally past each other.
Features: Major fault lines, frequent shallow earthquakes. No significant crust is created or destroyed.
Example: San Andreas Fault in California. While Nigeria does not have active transform boundaries, understanding this mechanism helps explain intra-plate stresses. 2.
4. Associated Phenomena and Landforms Internal processes manifest in various geological phenomena and create distinctive landforms. 2.4.
1. Volcanism Definition: The process by which magma, gases, and ash are expelled from the Earth's interior onto its surface through vents or fissures. Magma vs.
Lava: Magma is molten rock beneath the Earth's surface; lava is molten rock once it erupts onto the surface.
Types of Volcanoes: Shield Volcanoes: Broad, gently sloping cones built by successive eruptions of fluid basaltic lava (e.g., Hawaii).
Composite (Stratovolcanoes): Steep-sided, conical volcanoes built by alternating layers of lava flows, pyroclastic material, and ash (e.g., Mount Fuji). Associated with more explosive eruptions.
Dome Volcanoes: Formed by viscous, pasty lava that piles up around the vent (e.g., Mt. Pelee).
Caldera: A large, basin-shaped depression formed when the summit of a volcano collapses after a major eruption.
Volcanic Materials: Lava (basaltic, andesitic, rhyolitic), volcanic ash, volcanic bombs, lapilli, gases (steam, CO2, SO2).
Distribution: Most volcanoes occur along plate boundaries, especially convergent (subduction zones - "Ring of Fire") and divergent (mid-oceanic ridges), and at "hot spots" (areas of persistent volcanic activity far from plate boundaries, caused by mantle plumes).
Benefits of Volcanism: Fertile Soils: Weathering of volcanic rocks produces rich, fertile soils ideal for agriculture (e.g., coffee plantations in Cameroon, which is part of the Cameroon Volcanic Line extending into Nigeria).
Geothermal Energy: Heat from magma chambers can be harnessed to generate electricity.
Mineral Deposits: Ores of copper, gold, silver, lead, and zinc are often associated with volcanic activity. (e.g., Nigeria's tin and columbite deposits on the Jos Plateau are linked to ancient intrusive igneous activity).
Tourism: Volcanic landscapes, hot springs, and crater lakes attract tourists.
Hazards of Volcanism: Lava flows, ash fall, pyroclastic flows (fast-moving hot gas and rock), lahars (volcanic mudflows), toxic gases, tsunamis (if submarine eruption).
Nigerian Relevance: While Nigeria does not have currently active volcanoes, the Cameroon Volcanic Line (CVL) extends into Nigeria, influencing landforms like the Biu Plateau and creating ancient volcanic rocks. Understanding the CVL is important for regional geological context. 2.4.
2. Earthquakes Definition: A sudden and rapid shaking of the Earth's crust caused by the release of energy that has accumulated in rocks.
Causes: Primarily tectonic plate movements (release of accumulated stress along fault lines), but also volcanic activity, meteor impacts, and human activities (e.g., reservoir-induced seismicity, mining blasts).
Terminology: * Focus (Hypocentre): The point within the This section provides the essential content knowledge for the teacher to deliver the lesson effectively. 2.
1. Introduction to Earth's Internal Processes (Endogenic Processes) Earth's internal processes, also known as endogenic processes, are geological activities that originate from within the Earth. These processes derive their energy from the Earth's internal heat and lead to the deformation of the Earth's crust, resulting in the creation of major relief features such as mountains, plateaus, rift valleys, and oceanic trenches. They also cause phenomena like earthquakes and volcanic eruptions. 2.
2. The Internal Structure of the Earth The Earth is composed of several concentric layers, much like an onion. These layers differ in composition, density, temperature, and pressure. Understanding this structure is fundamental to comprehending internal processes.
Crust: The outermost solid layer of the Earth.
Thickness: Varies from 5-10 km under oceans (oceanic crust) to 30-70 km under continents (continental crust).
Composition: Primarily silicate rocks.
Oceanic Crust: Denser, mainly basaltic (rich in silica and magnesium - SIMA).
Continental Crust: Less dense, mainly granitic (rich in silica and aluminium - SIAL).
Temperature: Relatively cool, increasing with depth.
Mantle: The layer beneath the crust, extending to a depth of about 2,900 km.
Composition: Dense, hot, silicate rock, rich in iron and magnesium.
State: Mostly solid but behaves plastically or semi-molten in its upper parts (asthenosphere) due to high temperatures and pressures, allowing for slow convection.
Temperature: Ranges from approximately 500°C at the crust-mantle boundary to over 4,000°C near the core.
Key subdivisions: Lithosphere: Comprises the crust and the rigid uppermost part of the mantle. It is broken into tectonic plates.
Asthenosphere: The ductile, weaker layer within the upper mantle, below the lithosphere. It is semi-molten and allows the tectonic plates to move over it.
Core: The innermost layer of the Earth, primarily composed of iron and nickel.
Outer Core: Liquid layer, extending from 2,900 km to 5,150 km depth. Its convection currents are responsible for generating Earth's magnetic field.
Inner Core: Solid sphere, extending from 5,150 km to the Earth's centre (6,371 km). It is solid due to immense pressure, despite extremely high temperatures (up to 6,000°C).
Heat Source: The Earth's internal heat primarily comes from:
1. Residual heat: Heat left over from the planet's formation.
2. Radioactive decay: Heat generated by the decay of radioactive isotopes (e.g., uranium, thorium, potassium) within the mantle and core. 2.
3. Theory of Plate Tectonics Plate tectonics is the unifying theory in geology that explains the large-scale motion of Earth's lithosphere. The lithosphere is broken into about a dozen large, rigid plates (and many smaller ones) that move slowly over the ductile asthenosphere.
Mechanism of Plate Movement: Convection currents in the mantle. Hot, less dense material rises from the deeper mantle, spreads laterally beneath the lithosphere, cools, becomes denser, and sinks back down. This continuous cycle drags the tectonic plates along.
Evidence for Plate Tectonics:
1. Continental Drift (Alfred Wegener's theory): Jigsaw fit of continents: The coastlines of continents (e.g., South America and Africa) appear to fit together.
Fossil distribution: Identical fossil species found on widely separated continents (e.g., Mesosaurus in South America and Africa).
Similar rock types and mountain ranges: Matching geological structures and rock sequences across oceans (e.g., Appalachian Mountains of North America and Caledonian Mountains of Scotland and Scandinavia).
Paleoclimate evidence: Evidence of ancient glaciers in tropical regions and tropical swamp deposits in polar regions.
2. Seafloor Spreading (Harry Hess): Mid-oceanic ridges: Underwater mountain ranges where new oceanic crust is formed.
Magnetic striping: Symmetrical patterns of magnetic reversals in rocks on either side of mid-oceanic ridges, indicating new crust forms and spreads outwards.
Age of oceanic crust: Rocks are youngest near mid-oceanic ridges and progressively older further away. Global distribution of earthquakes and volcanoes: Concentrated along plate boundaries.
Types of Plate Boundaries:
1. Divergent Boundaries (Constructive): Plates move apart.
Features: Mid-oceanic ridges (e.g., Mid-Atlantic Ridge), rift valleys (e.g., East African Rift Valley), volcanism, shallow earthquakes. New crust is generated here.
Example in Nigeria's vicinity: The East African Rift System extends The hanging wall moves down relative to the footwall.
Associated Landforms: Rift valleys (graben), block mountains (horst).
Example: The East African Rift Valley is formed by extensive normal faulting.
Reverse Fault: Caused by compressional forces (pushing together). The hanging wall moves up relative to the footwall.
Associated Landforms: Associated with folded mountain belts, crustal shortening.
Thrust Fault: A type of reverse fault where the fault plane has a very low angle (less than 45°), causing the overlying block to be pushed a great distance horizontally.
Strike-Slip Fault (Transform Fault): Caused by shear forces (sliding past each other horizontally). Movement is primarily horizontal along the fault plane.
Example: San Andreas Fault.
Landforms: Block mountains (Horsts) and Rift Valleys (Graben) are prominent features associated with faulting. Escarpments can also form. * Nigerian Relevance: The Benue Trough is a prominent geological feature in Nigeria, a major rift system formed by ancient faulting and continental extension, although currently tectonically inactive. Understanding faulting helps explain the basin structure and associated sedimentary deposits. currently active volcanoes, the Cameroon Volcanic Line (CVL) extends into Nigeria, influencing landforms like the Biu Plateau and creating ancient volcanic rocks. Understanding the CVL is important for regional geological context. 2.4.
2. Earthquakes Definition: A sudden and rapid shaking of the Earth's crust caused by the release of energy that has accumulated in rocks.
Causes: Primarily tectonic plate movements (release of accumulated stress along fault lines), but also volcanic activity, meteor impacts, and human activities (e.g., reservoir-induced seismicity, mining blasts).
Terminology: Focus (Hypocentre): The point within the Earth where the earthquake rupture originates.
Epicentre: The point on the Earth's surface directly above the focus.
Seismic Waves: Energy released propagates as waves.
Body Waves: Travel through Earth's interior.
P-waves (Primary/Compressional): Fastest, travel through solids, liquids, and gases. Push-pull motion.
S-waves (Secondary/Shear): Slower, travel only through solids. Side-to-side motion.
Surface Waves: Travel along Earth's surface. Cause most damage.
Love waves: Horizontal shear motion.
Rayleigh waves: Rolling motion.
Measurement: Seismograph: Instrument that detects and records seismic waves.
Richter Scale: Measures the magnitude (energy released) of an earthquake. Logarithmic scale (each whole number increase represents a tenfold increase in amplitude and approximately 32 times more energy released).
Mercalli Intensity Scale: Measures the intensity (observed effects/damage) of an earthquake at a particular location. Uses Roman numerals (I-XII).
Distribution: Most earthquakes occur along plate boundaries, especially convergent and transform boundaries. Intra-plate earthquakes (within plates) also occur due to accumulated stress on ancient fault lines.
Hazards of Earthquakes: Ground shaking, liquefaction (saturated soil loses strength), landslides, tsunamis (seismic sea waves, if submarine earthquake), fires, structural collapse.
Nigerian Relevance: Nigeria is generally considered seismically stable as it is away from active plate boundaries.
However, minor tremors (e.g., in Abuja, Kaduna, Saki, Oyo State) have been reported. These are attributed to intra-plate stresses, reactivation of old fault lines, or sometimes related to human activities (e.g., large reservoir construction). While not devastating, understanding earthquake principles is crucial for building resilient infrastructure and emergency planning. 2.4.
3. Folding Definition: The bending and deformation of rock strata due to compressional forces. Occurs primarily at convergent plate boundaries where plates collide.
Key Terms: Anticline: An upfold or arched fold, where rock layers dip away from the fold axis (oldest rocks in the core).
Syncline: A downfold or trough-like fold, where rock layers dip towards the fold axis (youngest rocks in the core).
Fold Limb: The side of a fold.
Axial Plane: An imaginary plane that divides a fold as symmetrically as possible.
Fold Axis (Hinge Line): The line formed by the intersection of the axial plane with the bedding surface.
Types of Folds: Symmetrical, Asymmetrical, Overturned, Recumbent, Nappe (overthrust fold).
Landforms: Fold mountains are major features formed by intense folding (e.g., Himalayas, Alps, Atlas Mountains).
Nigerian Relevance: Nigeria's landscape features ancient fold structures, particularly in the North-Central Highlands and parts of the Benue Trough, which are relics of past tectonic activity. These are not active fold mountains but show evidence of ancient folding. 2.4.
4. Faulting Definition: The fracturing and displacement (slipping) of rock strata along a plane of weakness (a fault plane) due to tensional, compressional, or shear forces.
Key Terms: Fault Plane: The surface along which movement occurs.
Hanging Wall: The block of rock that lies above an inclined fault plane.
Footwall: The block of rock that lies below an inclined fault plane.
Types of Faults: Normal Fault: Caused by tensional forces (pulling apart). The hanging wall moves down relative to the footwall.
Associated Landforms: Rift valleys (graben), block mountains (horst).
Example: The East African Rift Valley is formed by extensive normal faulting.
Reverse Fault: Caused by compressional forces (pushing together). The hanging wall moves up relative to the footwall.
Associated Landforms: Associated with folded mountain belts, crustal shortening.
Thrust Fault: A type of reverse fault where the fault plane has a very low angle (less than 45°), causing the overlying block to be pushed a great distance horizontally. * *Strike-Slip
Connecting the topic to real-life situations helps students appreciate its relevance and practical value.
Resource Exploration and Management: Application: Understanding Earth's internal processes is fundamental to the exploration and extraction of mineral resources. Many valuable minerals (e.g., tin, columbite found on the Jos Plateau) are associated with igneous intrusions and ancient volcanic activity. Petroleum resources in sedimentary basins (like the Niger Delta) are often found in structures created by faulting and folding over geological time. Knowledge of plate tectonics and fault systems guides geologists in identifying potential sites for oil, gas, and solid mineral deposits in Nigeria.
Nigerian Context: Geologists and mining engineers in Nigeria use principles of structural geology (folding and faulting) to locate and exploit resources, contributing significantly to the national economy. The Benue Trough, for instance, is a major rift system with associated sedimentary basins that have potential for hydrocarbon exploration. Infrastructure Development and Disaster Preparedness: Application: Knowledge of seismic activity and fault lines is crucial for civil engineers and urban planners. While Nigeria experiences low seismicity, understanding where ancient fault lines exist (e.g., in the FCT, Kaduna, Saki regions) informs building codes and construction practices. In areas prone to earthquakes or volcanic activity (e.g., along the Cameroon Volcanic Line which is active just outside Nigeria's border), specific building standards (earthquake-resistant designs) and land-use planning are vital to minimize damage and loss of life.
Nigerian Context: Although major earthquakes are rare, minor tremors have occurred. Applying this knowledge helps in developing resilient infrastructure in Nigeria's urban centres. For instance, designing critical infrastructure like bridges, dams, and high-rise buildings to withstand potential (even minor) ground shaking is a practical application.
Furthermore, understanding volcanic hazards in the neighbouring Cameroon Volcanic Line helps Nigeria prepare for potential transboundary impacts like ash fall.
Agriculture and Soil Fertility: Application: Volcanic eruptions, despite their destructive power, often result in highly fertile soils over time. The weathering of volcanic ash and lava releases nutrient-rich minerals, making these areas highly productive for agriculture.
Nigerian Context: Regions like the Biu Plateau in Borno State, characterized by ancient volcanic activity, benefit from fertile soils derived from weathered basalt, supporting robust agricultural practices. Understanding this link allows for better land-use planning and agricultural practices in such areas, maximizing crop yields.